What to use as a vapor barrier?

I'm remodling my bathroom, it started out as a simple re-tiling job but once I got the old tile off I saw how bad the walls were underneath the tile and decided I had to rip it out to the studs. Anyway the old installation had no vapor barrier and no insulation in the outside walls. I've taken care of the insulation problem but now I'm not sure what to use as a vapor barrier. I've read a few different things, Redguard on the cement board, poly on the studs, roofing felt on the studs. Is there on set material that most people use? Thanks for the help.